In article ,
"Frank Dresser" wrote:

"Telamon" wrote in
message

.com...

[snip]


Some detector designs would use a DC bias on the diode to put it on
the edge of its liner region to improve its small signal
sensitivity. The optimum bias voltage will depend on the diode
characteristics.


There's a linear region in the usual model of a semiconductor diode
(a fixed voltage drop with a series resistance), but that model is
only an approximation. The other model, the square law model, is
also just an approximation, although it's supposed to be close enough
over small parts of the curve.

However, the diode doesn't have to be linear in order to have a
fairly linear diode detector circuit. Imagine we have a diode whose
forward resistance drops in a square law with the voltage. At 0.1V
the forward resistance is 1 meg. At 0.2V the forward resistance is
1K. At .0.3V the forward resistance is 32 ohms. At 0.4V the
resistance is 5.6V, and so on.

Now, let's put this nonlinear diode in series with a linear load
resistance and decide that the circuit is pretty much linear once the
diode resistance drops to 10% of the load resistance. Well, it's
obvious that diode detector circuits which work into higher
resistance loads will linearize themselves at lower voltages than
diode detectors which work into lower resistance loads.

Below a certain voltage, the diode's non linear characteristics will
dominate the detector. Low voltage signals will have much more of
their waveform in this funky reigion than high voltage signals, even
at the same modulation index.

So, as I see it, there's alot more to know about a diode detector's
audio distortion than only the modulation index. There's the actual
characteristics of the diode, the resistance of the load and the
signal voltage the detector is operating at.

There's also the RF filtering, which will tend to "sawtooth" the
audio a bit, much as the rectifier and capacitor do in a power
supply. There's also some resistances/capacitances in the AVC line.

But I could be wrong. If so, let me know!

I don't anything wrong with what you wrote but you seem to think that
the diode used makes no difference because you can make it up its
deficiencies with an amplifier whose input impedance and gain adjusts
for it. Basically that is true that you can use a less efficient diode
but you will have to provide higher signal levels to it and weak
signals will still be distorted due to compression. I suppose you could
use a logarithmic type amplifier following the detector in order to make
up for the compression.

If you look at the diode curves germanium has one of the better forward
current to input voltage ratios of several diode types. Not being a
radio designer my approach would be to use a diodes fairly liner region
with a better forward current to input voltage ratio where the least
distortion and compression would be due to it and therefor the least
needed correction to be made up for by a amplifier with a fixed
correction. Another reason to use a more efficient diode besides the
signal level power needed is the power the diode itself burns when you
bias the diodes with larger forward voltage junctions.

--
Telamon
Ventura, California